The purification treatment for effluents such as industrial waste water, animal sewage, and sewage water often employs activated sludge processes, which have high treatment efficiency. In particular, a process attracting attention is a membrane separation type activated sludge process (MBR process), which performs separation between treated water and sludge not by the conventional precipitation method, but with a microfiltration membrane (MF membrane) or an ultra filtration membrane (UF membrane). Examples of a purification treatment system employing this membrane separation type activated sludge process include a system that includes an aeration tank and a membrane separation tank as separate tanks, and a one-tank system in which a filtration membrane is immersed in a reactor.
The aeration tank is a tank where a large amount of microbes grown are used to capture and consume contamination substances that are mainly organic substances in effluent, to thereby purify the effluent. Flocs of such microbes having the capability of purifying effluent are referred to as activated sludge. The aeration means supplying of air to water to thereby supply oxygen. Some microbes require oxygen to live, and, in the activated sludge process, aeration is performed by supplying air to the aeration tank with a blower from the bottom portion of the aeration tank or by stirring the surface in the aeration tank.
The filtration membrane, which separates purified water (treated water) and activated sludge from each other in the aeration tank, is unavoidably subjected to clogging (fouling) due to adhesion of activated sludge to the surface of the filtration membrane. For this reason, it has been proposed that activated sludge adhering to the surface of the filtration membrane is removed by supplying air bubbles from beneath the filtration membrane and scrubbing the surface of the filtration membrane with the air bubbles (for example, refer to Japanese Unexamined Patent Application Publication No. 2010-253355).
To reduce clogging of the filtration membranes, it is necessary to regulate flux per unit area of the treated water that passes through the filtration membranes to less than or equal to a certain value. Accordingly, the above publication discloses an apparatus structure in which the waste water (raw water) can be temporarily stored in an adjusting tank and supplied to an activated sludge tank at a constant flow rate. When the inflow rate of the waste water exceeds a certain amount, the membrane separation type activated sludge treatment system according to the related art stores the excess waste water in the adjusting tank. Then, when the inflow rate of the waste water becomes lower than the certain amount, the membrane separation type activated sludge treatment system according to the related art supplies the waste water stored in the adjusting tank to the activated sludge tank together with the waste water that flows into the activated sludge tank. Thus, variation in the amount of waste water treated in the activated sludge tank per unit time is reduced.
Because variation in the amount of waste water treated in the activated sludge tank per unit time is reduced, the membrane separation type activated sludge treatment system according to the related art can be designed to have a maximum filtration performance lower than the maximum inflow rate of the waste water. Therefore, the total volume of the filtration membranes included in a filtration module to be installed can be reduced, and the operating energy consumption of the membrane separation type activated sludge treatment system according to the related art can be reduced accordingly.